The invention relates to a method of determining a display parameter in a picture tube which comprises a system for emitting a frequency-modulated electron beam, a modulated image pattern being produced on the display screen of the picture tube, and the display parameter being determined from the image pattern.
The invention also relates to a method of improving a picture display in a picture tube.
A method of the type defined in the opening paragraph is disclosed in the European Patent Application 0,281,195, corresponding to U.S. Pat. No. 4,814,669, in which a method of determining the position of an electron beam is described. The prior-art picture tube includes a colour selection electrode. The colour selection electrode has apertures. The modulation frequency of the electron beam differs from the modulation frequency of the colour selection electrode. Modulation frequency of the electron beam must here be understood to mean 2.pi. divided by the distance between maxima in the intensity of the electron beam. The modulation frequency of the colour selection electrode must be understood to mean 2.pi. divided by the spacing between the apertures in the colour selection electrode. A triad of phosphor dots or lines is present on the display screen for each aperture in the colour selection electrode. Consequently, the modulation frequency of the phosphor pattern, i.e. 2.pi. divided by the distance between consecutive phosphor elements of the same colour, is substantially equal to the modulation frequency of the colour selection electrode. A modulated picture pattern results on the picture screen having a frequency equal to the difference between the modulation frequency of the electron beam and the modulation frequency of the phosphor pattern. The position of the electron beam can be determined from this picture pattern. This method has the disadvantage that other display parameters, for example the accuracy of focusing on the picture screen cannot be determined. The simultaneous determination of more than one display parameters would save time and consequently costs.